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Int. Journal of Renewable Energy Development 6 (3) 2017: 273-281

Page | 273

© IJRED – ISSN: 2252-4940, October 15th 2017, All rights reserved

Contents list available at IJRED website

Int. Journal of Renewable Energy Development (IJRED)

Journal homepage: http://ejournal.undip.ac.id/index.php/ijred

The Effects of Different Roughness Configurations on

Aerodynamic Performance of Wind Turbine Airfoil and Blade

bBehzad Forouzi Feshalami ,1b,, Mohammad Hassan DjavareshkianaKamyar Jafari

Mashhad, Iran ,Eqbal Lahuri Institution of Higher Education Department of Mechanical Engineering,a

of Mashhad, IranDepartment of Mechanical Engineering, Ferdowsi University b

ABSTRACT. In this research, viscous and turbulent flow is simulated numerically on an E387 airfoil as well as on a turbine

blade. The main objective of this paper is to investigate various configurations of roughness to find a solution in order to mitigate

roughness destructive impacts. Hence, the sand grain roughness is distributed uniformly along pressure side, suction side and

both sides during the manufacturing process. Navier-Stokes equations are discretized by the finite volume method and are

solved by SIMPLE algorithm in the OpenFOAM software which is open source. Results indicated that in contrast with previous

studies, the roughness will be useful if it is applied on only pressure side of the airfoil. In this condition, the lift coefficient is

increased to 8.62% and 1.2% compare to the airfoil with rough and smooth sides, respectively. However, in 3-D simulation, the lift coefficient of the blade with pressure surface roughness is less than smooth blade, but still its destructive impacts are much

less than of both surfaces roughness and suction surface roughness. Therefore, it can be deduced that in order to reveal the

influence of roughness, the simulation must be accomplished in three dimensions.

Keywords: Roughness, wind turbine blade, aerodynamic, E387 airfoil, CFD

Article History: Received Jun 12th 2017; Received in revised form August 27th 2017; Accepted Oct 3rd 2017; Available online

How to Cite This Article: Jafari, K., Djavareshkian, M.H., Feshalami, B.H. (2017) The Effects of Different Roughness Configurations

on Aerodynamic Performance of Wind Turbine Airfoil and Blade. International Journal of Renewable Energy Develeopment, 6(3), 273-

281.

https://doi.org/10.14710/ijred.6.3.273-281

1. Introduction

Wind energy, which is one of the renewable

sources, has been significantly developed in recent

years. At the end of 2015, the wind turbine installed

capacity is reached 63,467 MW (Globalwindstatistics.

et al.). Surface roughness is one of the critical factors to

pull aerodynamic performance of wind turbine down.

Roughness can be considered as obstacles which are

settled into the viscous layer. These obstacles increase

interaction between fluid and solid. Consequently, it

can seriously affect the aerodynamic performance of

airfoil (Sagol, Reggio, & Ilinca 2013). Chakroun, Al-

Mesri, & Al-Fahad (2004) highlighted that stall angle

can be delayed when the surface is rough. Also, when

the roughness is located at trailing edge, it has the least

adverse impact on performance. Darbandi et al. (2014)

1 Corresponding author: [email protected]

showed that annual energy production is faced with a

25% reduction when roughness is exerted on wind

turbine blade. By performing experimental and

numerical research on a NACA63-618 airfoil, Walker et

al. (2014) revealed that roughness reduces the

maximum power coefficient to 0.34 while this coefficient is 0.42 for clean one. Effects of roughness height have been investigated

by many researchers. Wu, Li, & Li (2013) showed that

the performance of a wind turbine airfoil is very

sensitive to the roughness height of 0.6 𝑚𝑚. Also, 53% − 92% of suction surface and 44% − 88% of pressure surface are the most sensitive regions.

Soltani, Askari, & Sadri (2016) experimentally

revealed that aerodynamic efficiency is reduced and the

drag is increased for airfoil with roughness height of

Citation: Jafari, K., Djavareshkian, M.H., Feshalami, B.H. (2017) The Effects of Different Roughness Configurations on Aerodynamic Performance of Wind

Turbine Airfoil and Blade. Int. Journal of Renewable Energy Development,6(3), 273-281, doi : 10.14710/ijred.6.3.273-281

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0.5 𝑚𝑚. By performing experimental research, David et al. (2016) studied the effects of roughness on the

performance of NACA63-415 and SERIS814 airfoils.

Results indicated that increment of roughness height

has more influence on SERIS814 airfoil in order to

reduce maximum lift and lift to drag ratio than the

NACA63-415 airfoil. Bai et al. (2014) numerically

pointed out that the total pressure loss coefficient is

reached 129% for the rough blade in comparison with the smooth blade. Also, 𝑅𝑒 enhancement has unfavorable effects on the performance of the rough

blade. By performing experimental research on a 53°

leading edge sweep diamond wing, Hövelmann, Knoth,

& Breitsamter (2016) showed that roughness height

significantly affects the flow separation onset and the

emerging leading edge vortex. Khanjari,

Sarreshtehdari, & Mahmoodi (2017) concluded that

increment of roughness height from 0 to 0.5 𝑚𝑚 reduces the net power.

Environmental conditions, including dusty, rainy

and snowy weathers, can jeopardize aerodynamic

performance of wind turbine due to the increment of

the blade roughness (Zidane et al. 2016). Effects of

various 𝑅𝑒, roughness height and roughness shape in a dusty condition experimentally tested by Hummel et al.

(2005). They showed that increment of 𝑅𝑒 increases total pressure losses. They also revealed that at 𝑅𝑒 =1.2 × 106 and the surface roughness height of 11.8 𝜇𝑚, total pressure loss is faced with 40% enhancement in comparison with the smooth surface. Influence of dust

accumulation on performance of wind turbine

experimentally examined by Khalfallah & Koliub

(2007). They showed that by enhancement of dust on

wind turbine blades, drag increases while lift reduces.

This can finally lead to reduction of power production.

They also mentioned that this reduction depends on

airfoil type, 𝑅𝑒, sand size based on boundary layer thickness and nature of roughness. Homola et al. (2010)

numerically analyzed impacts of temperature and

droplet size on the aerodynamic performance of a wind

turbine blade. They expressed that the temperature

and droplet size don’t have significant effect on flow

separation of the blade at 5° angle of attack. However, by increasing the angle of attack to 15°, reduction of temperature and enhancement of droplet size,

respectively, reduces and increases flow separation. By

simulating a NACA0012 airfoil under heavy rain

condition, Douvi & Margaris (2012) proved that rain

drop increases drag and also leads to lift reduction.

They also showed that this variation can be intensive

at higher 𝑅𝑒 and angle of attack. El-Din & Diab (2016) numerically investigated behavior of various airfoils

against created roughness by erosion of sands. They

showed that NREL airfoils have higher resistance to

erosion than the other types, namely NACA and DU, at

higher angle of attack.

There are lots of numerical methods to investigate

the impacts of roughness on flow behavior. Meng-

Huang & William (2009) examined ability of a new

second-order closure of the rough wall layer model in

order to simulate a rough NACA0012 airfoil. Results

showed that this method can properly predict the

aerodynamic characteristics of flow before separation.

Capability of two models, low Reynolds number shear

stress transport model and 𝛾 − 𝑅𝜃 shear stress transport model, to simulate a rough NACA0012 airfoil

is studied by Liu & Qin (2014). Results indicated that

first method is able to simulate the roughness of

surface properly while the other model is not. CFD

methods have been widely used to evaluate

aerodynamic performance of wind turbine. Munduate

& Ferrer (2009) evaluated the capability of panel

method and CFD technique in order to predict wind

turbine blade life cycle under different conditions.

Results revealed that panel method is not reliable to

simulate roughness effects of fully turbulent flows. In

contrast, obtained results by CFD methods have been

had acceptable agreement with experimental data.

Performance of Wind turbine blade is sensitive to

roughness patterns. Timmer & Schaffarczyk (2004)

experimentally showed that the adverse impacts of

carborundum 60 roughness are more than of zigzag

tape roughness on performance of a DU97-w-300

airfoil. However, increasing 𝑅𝑒 mitigates these undesirable impacts. Soltani, Birjandi, & Seddighi

Moorani (2011) experimentally proved that zigzag and

strip tape roughness patterns have less effect on flow

characteristics than that of distributed contamination.

To summarize, all of the studies have been revealed

that existence of roughness for any reason decreases

the aerodynamic efficiency. Furthermore, increasing

angle of attack and roughness height can also intensify

roughness adverse impacts. It was also shown that the

results are very sensitive to the roughness pattern. The

main objective of this paper is to investigate effects of

various roughness configurations on wind turbine

aerodynamic performance. Hence, E387 airfoil and

blade, which can be used in wind turbines, are

considered to do simulations (Krishnaswami 2013). The

sand grain roughness is distributed uniformly along

pressure side, suction side and both sides during

manufacture process. The authors aimed to find the

solution in order to mitigate the disadvantages of

roughness on the performance of a wind turbine.

Moreover, simulations are conducted in two and three

dimensions to reveal why the results in two dimensions

may lead to an egregious inaccuracy.

2. Numerical method

2.1. Governing equations and discretization

Conservation laws, namely continuity and

momentum, are governing equations which are

represented as follows:

(1)

𝜕𝜌

𝜕𝑡+ 𝛻 . 𝜌𝑉 = 0

(2)

𝜕

𝜕𝑡(𝜌𝑢𝑖) +

𝜕

𝜕𝑥𝑗(𝜌𝑢𝑖𝑢𝑗) = −

𝜕𝑃

𝜕𝑥𝑖+

𝜕𝜏𝑖𝑗

𝜕𝑥𝑗+ 𝐹𝑖


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where 𝜌 and 𝑉 are density and velocity vector of fluid. 𝑢𝑖 and 𝑢𝑗 are velocity in x and y directions. Also, P, 𝜏𝑖𝑗 and

𝐹𝑖 show, respectively, pressure, stress tensor and internal force.

Stress tensor can be expressed by:

where 𝜇 is viscosity. To simulate turbulence effects, 𝑆𝑆𝑇 𝑘 − 𝑤 model is applied in these equations, where k and ω indicate turbulent kinetic energy and energy loss

rate. Also 𝑌, 𝐷, 𝑆, 𝐺 and Г are terms of loss, propagation length, source, production rate and diffusion,

respectively

Wall function law, which is shown in eq. (6), is

modified for rough walls in eq. (7) (Ioselevich &

Pilipenko 1974):

where 𝑘 is Von Karman constant and is equal to 0.04. 𝑢𝑝 is the center velocity of first cell near surface and 𝑦𝑝

indicates the distance between point p and rough wall.

𝑐𝜇 is experimental constant and is equal to 0.09. Also,

∆𝐵 is the roughness function and is related to roughness types, including uniform sand, rivets,

threads ribs, mesh-wire and etc., and roughness size. It

should be noted that there is not general roughness

function valid for all roughness types. Roughness

function is related to normalized roughness average

height that is shown by 𝑘+. This parameter can be calculated as follows:

(9)

𝐾𝑠+ =

𝜌𝑅𝑎𝑢∗

𝜇

where 𝑘 is the turbulent kinetic energy of the cells that is located near the wall. In this paper, the Arithmetic

Average Height, 𝑅𝑎, is used to express roughness parameter (Gadelmawla et al. 2002). This can be

defined as follows:

(10) 𝑅𝑎 =

1

𝑛∑|𝑦𝑖|

𝑛

𝑖=1

where iy indicates heights of roughness. According to

this equation, 𝑅𝑎 is average absolute deviation of roughness oscillations. In this research, various values

of 𝑅𝑎 are examined to show its effects on aerodynamic performance.

There are three distinct regimes for rough wall: (1)

smooth regime with 𝐾𝑠+ < 3~5 (2) transient regime with

3~5 < 𝐾𝑠+ < 70~90 and (3) fully rough regime with

𝐾𝑠+ > 70~90 (Cebeci & Bradshaw 1977). Values of

roughness function for transition and fully rough

regimes can be acquired by eqs. (11) and (12),

respectively: 𝐶𝑠 is the roughness coefficient and depends on the type of roughness. In this research,

content of 0.5 is considered for uniform sand grain roughness and more values can be used for non-

uniform one (Levin, Semin, &Klyukin 2014). Eqs. (11)

and (12) are used for sand grain roughness and similar

types of uniform roughness elements. Navier-Stokes

equations are discretized by the finite volume method

and are solved by the SIMPLE implicit method.

2.2. Mesh and boundary condition

In order to produce proper mesh, center height of first

cell in the vicinity of the wall must be equal or more

than of roughness average height. It should be noted

that roughness effect cannot be observed in the

simulation process when the height of first cell is

considered much more than of roughness average

height (Natarajan & Hangan 2009). Logarithmic region

is suitable for computational domain to evaluate

roughness effect. Thus, 𝑌+ is restricted between 30 and 500 (Blocken, Stathopoulos, &Carmeliet 2007). Contents of 10𝑐 and 25𝑐 are considered as width and

(4)

𝜕

𝜕𝑡(𝜌𝑘) +

𝜕

𝜕𝑥𝑖(𝜌𝑘𝑢𝑖)

=𝜕

𝜕𝑥𝑗(𝛤𝑘

𝜕𝑘

𝜕𝑥𝑗) + �̃�𝑘 − 𝑌𝑘 + 𝑆𝑘

(5)

𝜕

𝜕𝑡(𝜌𝜔) +

∂

∂𝑥𝑖(𝜌𝜔𝑢𝑖)

=𝜕

𝜕𝑥𝑗(𝛤𝜔

𝜕𝜔

𝜕𝑥𝑗) + 𝐺𝜔 − 𝑌𝜔 + 𝐷𝜔

+ 𝑆𝜔

(3) 𝜏𝑖𝑗 = [𝜇 (

𝜕𝑢𝑖

𝜕𝑥𝑗+

𝜕𝑢𝑗

𝜕𝑥𝑖)]

(11)

∆𝐵 =1

𝑘ln (

𝐾𝑠+ − 2.25

87.75+ 𝐶𝑠𝐾𝑠

+)

× sin{0.4258(ln 𝐾𝑠+ − 0.811)}

(12)

∆𝐵 =1

𝑘ln(1 + 𝐶𝑠𝐾𝑠

+)

(6)

𝑢+ =1

𝐾𝑙𝑛(𝑦+) + 𝐵

(7)

𝑢𝑝𝑢∗

𝜏ɷ𝜌⁄

=1

𝑘𝑙𝑛 (𝐸

𝜌𝑢∗𝑦𝑝

𝜇) − ∆𝐵

(8)

𝑢∗ = 𝑐𝜇1

4⁄ 𝑘1

2⁄

Figure 1. Computational region and boundary conditions for airfoil



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length of computational domain of airfoil, which is

shown in Fig. 1.

As demonstrated in Fig. 2, the mesh is as structured

type. On the other hand, unstructured mesh is used for

wind turbine blade due to the complexity of geometry.

Computational domain and Periodic boundary

condition are shown in Fig. 3 for wind turbine blade

with radius of 17 𝑚.

2.3. Mesh independency and validation

For airfoil simulation, mesh with cell numbers of

78,000, 106,000 and 159,200 are generated to investigate mesh independency. Pressure

coefficient, 𝐶𝑝, for aforementioned meshes are

illustrated in Fig. 4 𝑎𝑡 𝑅𝑒 = 4.6 × 105 and 0° angle of attack. Moreover, Grid solution study for turbine blade

has been performed to ensure independency of the

results from the mesh. Hence, 𝐶𝑝 at 0.95𝑐, wind velocity

of 20 𝑚 𝑠⁄ and angular velocity of 6.17 𝑟𝑎𝑑

𝑠⁄ are depicted in Fig. 5 for various cell numbers. According

to these figures, by increasing mesh cell numbers from

106,000 to 152,200 for the airfoil and from 1,029,597 to 1,467,876 for the blade, although computational cost increases, the accuracy is almost constant. Therefore,

the meshes with cell numbers of 106,000 and 1,029,597 are employed for the airfoil and blade, respectively.

Fig. 2. Mesh around E387 airfoil

Fig. 3. Computational region and boundary conditions around wind turbine blade

Fig. 4. Mesh independency based on pressure coefficient distribution for the E387 airfoil at 𝛼 = 0

Fig. 5. Mesh independency based on pressure coefficient distribution at 95% of blade radius

Fig. 1. Computational region and boundary conditions for airfoil


Page | 277


Due to the lack of experimental research about

present airfoil, firstly simulation is conducted on a

S809 airfoil and is compared with experimental data of

Somers (1989) in Fig. 6. Then, lift coefficient of present

research for the E387 airfoil is compared with that of

Bidarouni & Djavareshkian (2013) in Fig. 7 at 𝑅𝑒 =

4.6 × 105 , 0, 5 and 10° angles of attack, 𝜌 = 1.225 𝑘𝑔

𝑚3⁄

and 𝜇 = 1.7894. It should be noted that in this figure, 𝑘 − 𝜀 turbulence model is used to compare with that of Bidarouni & Djavareshkian (2013) in order to prove

trueness of present simulation. However, this model

doesn’t have acceptable performance in high angles of

attack due to generation of reversible flows. Therefore,

𝑆𝑆𝑇 𝑘 − 𝑤 model is applied to do simulations(Versteeg & Malalasekera 2007). Also, power coefficient of

current study is compared with that of Saber &

Djavareshkian (2014) in Fig. 8 for various wind

velocities. Additionally, In order to prove trueness of

roughness simulation, lift coefficient of a NACA630-

430 airfoil at 𝜌 = 1.225𝑘𝑔

𝑚3⁄ , 𝜇 = 1.7894

𝑘𝑔𝑚. 𝑠⁄ , 𝑅𝑒 =

1.6 × 106 and 𝑅𝑎 = 0.5, 1 𝑚𝑚 is compared with data of Ren & Ou (2009). The difference in results, which are

tabulated in Table 1, can be justified as discrepancy of

two studies in mesh generation.

Table 1.

Comparison of the lift coefficient of present simulation with that

of Ren and Ou (Ren & Ou 2009) for the NACA630-430 airfoil

3. Results and discussion

Firstly, the airfoil with different roughness

average heights of 0, 0.06, 0.1, 0.3, 0.5, 0.8 and 1 𝑚𝑚 is simulated numerically at 𝑅𝑒 = 1.6 × 106 and 𝛼 = 5°. It should be noted that the simulation is accomplished

at first on the smooth airfoil and then the mentioned

roughness average heights are exerted to both sides

of the airfoil. As demonstrated in Figs. 9 and 10, by

increment of roughness average height, lift

coefficient diminishes while the drag increases. But

after a specific height, there is not a significant

alteration in these coefficients.

Lift coefficient of

present simulation

Lift coefficient of

Ren & Ou (2009)

𝑹𝒂 (mm)

0.44 0.48 0.5

0.41 0.46 1

Fig. 6. Comparison of pressure coefficient distribution between present study and experimental data of Somers (1989) at 50% of the S809 blade

Fig. 7. Comparison of present simulation with numerical results of Bidarouni & Djavareshkian (2013) for E387 airfoil

Fig. 8. Comparison of present simulation with numerical results of Saber & Djavareshkian (2014) for blade



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As listed in Table 2, Reduction in pressure difference

can justify decrement of aerodynamic efficiency.

However, based on Table 3, it is observed that adding

the roughness to one side of the airfoil can lead to

different results. In the other words, by exerting

roughness to only pressure side, lift force is faced with

8.62% enhancement in comparison with the airfoil with rough sides. Also, there is an 1.2% improvement in lift coefficient in comparison with the airfoil with smooth

sides because as demonstrated in Figs. 11 and 12, the

roughness reduces the flow velocity in the vicinity of

the pressure side. Therefore, the positive pressure

difference is increased much more than of other

conditions. In addition, roughness not only doesn’t have

destructive impacts but also increases aerodynamic

performance.

Table 2.

Lift and drag coefficients of airfoil with both sides roughness at

𝛼 = 5° and 𝑅𝑎 = 0, 0.8 𝑚𝑚

In contrast, roughness on only suction side has

adverse impacts. Because in this condition, roughness

is the factor to increase boundary layer thickness and

leads to movement of the separation point toward the

leading edge (see Fig. 13). Moreover, based on Figs. 11

and 12, roughness plays an important role to change

aerodynamic and pressure coefficients when it is

exerted to 0.6𝑐 of airfoil from leading edge. However, by reaching the trailing edge, impacts of roughness are

negligible because this region is located within the

turbulent flow. Additionally, the pressure difference

between upper and lower sides at leading edge is

increased than the other locations.

To investigate the validity of mentioned

configurations on turbine blade, the roughness with

average heights of 0, 0.1, 0.2 and 0.3 𝑚𝑚 are applied on surfaces of the turbine blade. Output torque of rough

blade is less than of smooth blade. Therefore, it causes

a reduction of energy production (Table 4 and Fig. 14).

Energy production of wind turbine is generally

increased by increment of tip speed ratio of the blade.

Based on Fig. 15, when roughness is added to this

blade, energy production will be reduced in comparison

with smooth blade because in the rough blade, the

possibility of separation increases by enhancement of

wind velocity and subsequently the flow will be less in

touch with blade surface.

Drag (N) Lift (N) 𝑹𝒂 (mm)

6.37 352.89 0

11.04 328.29 0.8

Fig. 9. Lift coefficient for various roughness average heights at 𝛼 = 5°

Fig.10. Drag coefficient for various roughness average heights at 𝛼 = 5°

Fig. 11. Pressure coefficient distribution along the airfoil sides for different roughness average heights at 𝛼 = 5°


Page | 279


Table 3.

Lift and drag coefficients for different roughness

configurations of airfoil at 𝛼 = 5° and 𝑅𝑎 = 0.8 𝑚𝑚

\

As illustrated in Fig. 16, in contrast with airfoil, the

lift force of blade with only pressure surface

roughness is less than of blade with smooth surfaces.

Nevertheless, roughness on the only pressure

surface of the blade has less energy production

reduction than suction surface and still can be better

solution than the blade with rough surfaces. This is

that reason which leads to difference between two

and three dimensional simulations.

Table 4.

Output torque of turbine blade for different Roughness

average heights

Two dimensional simulation of Roughness on the only pressure side increased the lift force more than of

airfoil with smooth sides while adverse results are

obtained in three dimensional simulation. Because

separation point on wind turbine blade at each radius

is different due to the existence of pitching angle.

Therefore, it can be concluded that in order to achieve

accurate results, the simulations must be accomplished

in three dimensions.

Reduction percentage

of output torque (%)

Output

torque (N-m)

𝑹𝒂 (mm)

- 60953.37 0

4.82 58013.07 0.1

7.05 56650.36 0.2

8.60 55710.48 0.3

Lift force (N) Lift Coefficient Roughness

Configuration

351.96 0.83 Smooth

324.76 0.77 Suction side

356.62 0.84 Pressure side

Fig. 12. Distribution of pressure coefficient along airfoil sides for three roughness configurations at 𝛼 = 5° and 𝑅𝑎 = 0.8 𝑚𝑚

Fig. 13. Boundary layer thickness for suction side of airfoil at 𝑥/ 𝑐 = 0.35 , 𝑅𝑎 = 0, 8 𝑚𝑚 and 𝛼 = 5°

Fig. 14. Variations of turbine Power coefficient versus roughness average height at wind speed of 20 m/s

Fig. 15. Variations of turbine power coefficient for various tip speed ratio at 𝑅𝑎 = 0, 0.3 𝑚𝑚



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4. Conclusion

Various roughness configurations were simulated numerically on an E387 airfoil and blade. The main

objective of this paper was to choose the best

configuration in order to mitigate destructive impacts

of roughness. Navier-Stokes equations were discretized

by the finite volume method and were solved by

SIMPLE algorithm. The main findings of present study

can be summarized as follows:

• When roughness is added to the only pressure

side of the airfoil, the lift force faced with 8.62% enhancement in comparison with the airfoil

with rough sides. However, the lift force of

blade with only pressure surface roughness is

less than of smooth surface. Nevertheless,

roughness on the only pressure surface of the

blade has less energy production reduction

than suction surface and still can be a better

solution than the blade with rough surfaces.

• By applying the roughness on the only suction

side of the airfoil, lift and drag are,

respectively, reduced and increased due to the

increment of boundary layer thickness and

movement of transient point toward the airfoil

leading edge.

• Output power is reduced by applying

roughness on turbine blades.

• Roughness plays an important role to change

aerodynamic and pressure coefficients when it

is exerted to 0.6𝑐 of airfoil from the leading edge. However, by reaching the trailing edge,

impacts of roughness are negligible because

this region is located within the turbulent flow.

• There is no alteration in aerodynamic

performance higher than of critical roughness

average height.

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